1. Field of the Invention
The present invention relates to an optical disc drive.
2. Description of the Prior Art
One remarkable feature of optical disc devices and discs such as DVDs and Blu-ray Discs (abbreviated as “BDs”) which are compliant with specifications is that recording media are exchangeable and that it is possible to interchange the discs between different kinds of machines and to perform recording on and reading out from the discs. However, surfaces of recorded discs get scratched and dirty as a matter of practice and consequently, readout becomes extremely difficult in some cases. While the scope of the present invention is not limited to BDs, description given below is directed to BDs and terminology used herein is also based on those used for BDs.
For a disc which is not in good condition due to the presence of a defect, grime and the like, measures are taken not to have a read error to the utmost extent at the time of readout. For example, when there is grime on the surface of a disc by which read signals are mostly blocked, it is possible to make the influence due to the defect to the minimum by employing a defect detection technology like described in Japanese Patent Application Laid-open No. 2003-30850. It has been known for those skilled in the art that similar technologies are generally used for optical discs. As shown in FIG. 2, an outline of the above is that the top envelope of a readout signal is monitored and that a defect detection signal is outputted when the amplitude is less than or equal to a threshold for more than a certain period of time. While the defect detection signal is outputted, the controlling of tracking or focusing is held, and a phase locked loop (abbreviated as a “PLL”) of a readout signal processing system is also held, whereby these unfavorable operations caused by those defects are prevented from being performed so that its influence is retained to the minimum.
When a read error occurs except for the time of a stream read or the like, a read retry is in general performed to obtain data of a recording unit block (abbreviated as a “RUB”) corresponding to the error. Heretofore, when performing a read retry, it has been performed according to a retry parameter list prepared in advance without identifying factors of the read error. Consequently, an effective read retry suitable against a cause of the read error is not necessarily performed.
Apart from local factors such as a defect and the like on a disc, there are phenomena that deteriorate the read capability over a considerably large area due to the structure of the disc such as an inter-layer interference of a dual layer disc. FIG. 3 is a view for showing an example in which a readout signal is disturbed by an inter-layer interference. This is the example in which a readout is performed on an L1 layer, i.e. a layer close to a surface, of a rewritable dual layer Blu-ray disc. It can be seen from the drawing that both top and bottom envelopes, which are supposed to be approximately flat, are being subjected to a large external disturbance due to an inter-layer interference. When reading out the L1 layer, a reading light is focused on the layer. Part of the reading light transmits the L1 layer, and it is reflected on the L0 layer, thereof reaching a photodetector of an optical head. Since the lights from both the L0 and L1 layers reach the photodetector at the same time, interference occurs between the lights. Moreover, a distance between the L0 layer and the L1 layer is slightly different at every position on a disc in general. When reading out the disc under such a condition, an interference pattern caused by the lights from the both L0 and L1 layers on the photodetector changes with time. As a result, a disturbance of a readout signal as shown in FIG. 3 occurs. When the disturbance of the signal occurs as shown in FIG. 3, a signal recorded on the part of disturbance cannot be correctly decoded as in the case of the defect, hence resulting in a burst error. In the case of the example shown in FIG. 3, a burst error having a length of several hundreds bytes occurs. Such a length brings no problem in readout so long as the capability of an error correction code of a BD system is concerned. However, when reading an area where the distance between layers changes rapidly in a tangential direction of the disc, the state of the interference on the photodetector changes more drastically than otherwise. Accordingly, signal disturbances as shown in FIG. 3 appear more frequently, thus plural of them will appear within a RUB. Under such a circumstance, a probability in which read errors occur is no longer negligible.